Single Bit and Multi-Bit Flash Memory Cells
Flash memory devices have been known for many years. Typically, each memory cell within a flash memory device stores one bit of information. The traditional way to store a bit in a flash memory cell has been by supporting two states of the memory cell. One state represents a logical “0” and the other state represents a logical “1”.
In a flash memory cell, the two states are implemented by having a floating gate situated above the cell's channel (the area connecting the source and drain elements of the cell's transistor), and having two valid states for the amount of charge stored within the floating gate. Typically, one state is with zero charge in the floating gate and is the unwritten state of the cell after being erased (commonly defined to represent the “1” state) and the other state is with some amount of negative charge in the floating gate (commonly defined to represent the “0” state). Having negative charge in the gate causes the threshold voltage of the cell's transistor (i.e., the voltage that has to be applied to the transistor's control gate in order to cause the transistor to conduct) to increase. It is possible to read the stored bit by checking the threshold voltage of the cell. If the threshold voltage is in the higher state then the bit value is “0” and if the threshold voltage is in the lower state then the bit value is “1”. Actually there is no need to accurately read the cell's threshold voltage. All that is needed is to correctly identify in which of the two states the cell is currently located. For this purpose it is sufficient to compare the threshold voltage of the cell to a reference voltage that is between the two states, and to determine if the cell's threshold voltage is below or above the reference value.
FIG. 1A (prior art) shows graphically how this works. Specifically, FIG. 1A shows a distribution of the threshold voltages of a large population of cells. Because the cells in a flash device are not exactly identical in their characteristics and behavior (due to, for example, small variations in impurity concentrations or defects in the silicon structure), applying the same programming operation to all of the cells does not cause all of the cells to have exactly the same threshold voltage. Instead, the threshold voltage is distributed as shown in FIG. 1A. Cells storing a value of “1” typically have a negative threshold voltage, such that most of the cells have a threshold voltage close to the central voltage value of the left peak (labeled 1) of FIG. 1A, with fewer cells having threshold voltages lower or higher than the central voltage of the left peak. Similarly, cells storing a value of “0” typically have a positive threshold voltage, such that most of the cells have a threshold voltage close to the central voltage of the right peak (labeled 0) of FIG. 1A, with fewer cells having threshold voltages lower or higher than the central voltage of the right peak.
In recent years, a new kind of flash device has appeared on the market, using “Multi Level Cells” (MLC). The term “Multi-Level Cell” is misleading because flash memory with a single bit per cell uses multiple i.e., two levels, as described above. Therefore, the term “Single Bit Cell” (SBC) is used hereinafter to refer to a memory cell of two levels and the term “Multi-Bit Cell” (MBC) is used hereinafter to refer to a memory cell of more than two levels, i.e., more than one bit per cell. The most common MBC flash memories at present are ones with two bits per cell, and therefore examples are given below using such MBC memories. It should however be understood that the present invention is equally applicable to flash memory devices that support more than two bits per cell. A single MBC cell storing two bits of information is in one of four different states. As the cell's “state” is represented by the cell's threshold voltage, an MBC cell supports four different valid ranges for the cell's threshold voltage. FIG. 1B (prior art) shows the threshold voltage distribution for a typical MBC cell of two bits per cell. As expected, FIG. 1B has four peaks, each peak corresponding to one state. As for the SBC, each state is actually a voltage range and not a single voltage. When reading the cell's contents, the cell's threshold voltage must be correctly identified in a definite voltage range. For a prior art example of an MBC flash device see U.S. Pat. No. 5,434,825 to Harari that is included by reference for all purposes as if fully set forth herein.
A cell designed for MBC operation e.g., in four states is typically operable as an SBC cell with two states. For example, Conley et al. in U.S. Pat. No. 6,426,893 incorporated by reference for all purposes as if fully set forth herein, disclosed the use of both MBC and SBC modes within the same device, and selecting certain parts of the device to operate with highest density in MBC mode, while other parts are used in SBC mode to provide better performance.
MBC devices provide a significant cost advantage. An MBC device with two bits per cell requires about half the area of a silicon wafer than an SBC of similar capacity. However, there are drawbacks to using MBC flash. Average read and write times of MBC memories are longer than of SBC memories, resulting in worse performance. Also, the reliability of MBC is lower than SBC. The differences between the threshold voltage ranges in MBC are much smaller than in SBC. Thus, a disturbance in the threshold voltage (e.g., leakage of stored charge causing a threshold voltage drift or interference from operating neighboring cells) that are insignificant in SBC because of the large gap between the two states, may cause an MBC cell to move from one state to another, resulting in an erroneous bit. The end result is a lower performance specification of MBC cells in terms of data retention time or the endurance of the device to many write/erase cycles.
Another ramification of the lower reliability of MBC devices compared to SBC devices is the required level of error correction. Manufacturers of SBC NAND flash devices typically advise users to apply an Error Correction Code (ECC) capable of correcting 1 bit error in each page of 512 bytes of data. But data sheets of MBC NAND flash devices typically advise applying an ECC capable of correcting 4 bit errors in each page of 512 bytes of data. For pages of size 2048 bytes such as in the case of NAND devices known as “large block devices”, the suggestion is to apply error correction per each portion of 512 bytes of the page. The present invention applies to all types of flash devices, regardless of page size. In this application the term “N-bit ECC” refers to an ECC scheme capable of correcting N bit errors in 512 bytes of data, regardless if the 512 bytes are the size of one page, less than one page, or more than one page.
Testing of Flash Memory Dies
Flash memory dies require extensive testing before being put to use in a product. This is especially true for NAND flash. One reason for this is that the flash device might have bad blocks that should not be used. A block is the smallest chunk of cells that can be erased in a single erase operation, and it typically contains many pages, where a page is the smallest chunk of cells that can be written in a single write operation. If a block cannot be reliably erased to the all “1” state or if one or more pages of the block cannot be reliably programmed, then the block should be either replaced by another block by physically trimming the die or be marked as a bad block so that software writing to the device will be able to avoid using it.
A given flash memory die may be subjected to other tests, but the testing for bad blocks is typically the most time consuming This is because testing for bad blocks typically involves writing to each and every page and erasing each and every block of the device. The writing and erasing is typically repeated more than once in order to make the bad blocks testing under different patterns of written data, under different temperatures, or under other variations of testing parameters.
Testing During the Wafer Sort Stage of Manufacturing
In some cases the full testing of a flash memory die is completed while the die is still part of the silicon wafer in which it was manufactured, and before the wafer is separated into multiple dies. This stage is typically called the “wafer sort” stage. Such dies that completed their testing in the wafer sort stage are typically called Known Good Dies (“KGD”). The use of KGD flash memory dies is advantageous when the flash memory die is assembled into a Multi-Chip Package (“MCP”) with an expensive second die such as a microprocessor. If the memory dies are not fully tested before the MCP assembly, then their testing is completed after the memory and microprocessor are assembled together into one device. Then if some of the memory dies are found defective at the MCP-stage testing, those MCP devices have to be scrapped. This causes the loss of not only the memory die (that would have been scrapped anyway), but also the expensive microprocessor die and the cost of packaging.
However, manufacturing KGD flash memory dies is not simple. The test equipment used for the wafer sorting stage is complex and expensive, using such equipment for long tests makes the costs of testing high. Also, bad blocks testing is not well suited for execution of the tests on many dies in parallel at the wafer sort stage. Unlike typical signal timing tests where all tested dies in the wafer go through exactly the same test sequences and the result of the test is a “go/nogo” decision for each die, in bad blocks testing each die might have bad blocks in different locations, and exhibit different timing while testing, and therefore the testing equipment should handle each die separately. This further complicates bad blocks testing at wafer sort stage and makes it more costly.
One other factor is even more significant in further increasing the costs of bad blocks testing at wafer sort stage. Many flash memory manufacturers require the tests to be done in temperatures different from room temperature. This means the wafer sort testing equipment should be able to conduct the testing while the tested wafer is kept in a specified temperature. This further increases the costs of the testing equipment and consequently the testing costs per die.
Because of the above, most flash memory dies are not manufactured as KGD. Instead the testing of the flash devices is divided into two parts. In the wafer sort stage only minimal tests are done, with the goal being to identify dies that are clearly bad and can be immediately discarded. Then the wafer is cut into separate dies and each die is packaged into the type of device package in which it will be sold. For NAND flash devices this is typically a TSOP, BGA or an LGA package, packages which are commonly used in electronic assemblies and Printed Circuit Boards (PCB). The rest of the testing, including the time-consuming bad blocks testing, is done on the packaged flash devices during a later stage (i.e., after the wafer sort stage) of manufacturing. Thus, any packaged devices are installed on a testing board (i.e., within the device-manufacturing facility), and the remaining tests are then executed. When testing packaged devices there is no need for expensive probing equipment, like the type needed for wafer sorting, as the flash devices can be conveniently handled and interfaced to, unlike the wafer sorting stage where access to the dies within the wafer is difficult.
However, even if packaged in a convenient package, testing of NAND flash devices for bad blocks is quite an expensive task. The reason for this is that due to the requirement for separate handling of each device (as explained above) the testing should be performed using expensive memory testers, which are capable of testing only a limited number of devices at the same time (typically about 100), and since the test time for each device is significant, so is the cost of testing. This is illustrated as a block diagram in FIG. 2.
Thus, referring to FIG. 2, it is noted that a flash memory device 110A to be tested is coupled to the memory tester 106. The flash memory device 110A includes a flash controller (including a processor 104) residing on a flash controller die 102, and a flash memory (including a plurality of memory cells) residing on one or more flash memory dies 100. Thus, the flash controller and the flash memory are on distinct dies. The memory tester 106 includes a processor 108. A testing program is executed by the memory tester 106 (in some examples, the testing program is also stored in non-volatile memory of the memory tester 106). The testing program, which is operative to test the flash memory die 100 (for example, individual memory cells of the flash memory die 100) is executed by the general purpose processor 108 which resides within the memory tester. Within FIG. 2, the processor 108 which executes the test program is marked with an asterisk. Although only a single flash memory device is depicted in FIG. 2, it is understood that typically, a plurality of devices (as mentioned above, typically about 100) are tested together in a batch (i.e., substantially simultaneously).
In general, it is noted that testing of MBC flash devices takes longer than testing SBC flash devices. This is especially true for bad blocks testing. This is because flash operations, and especially write operations, are much slower in MBC flash. Bad blocks testing requires many such operations, as typically each page in the device is written multiple times during the test. As the number of bits per cell in the MBC flash gets higher, the write operations become slower. So testing an SBC flash device is faster than testing a two-bits-per-cell MBC flash device, which in turn is faster than testing a four-bits-per-cell MBC flash device (assuming all devices have the same number of cells). With the increased use of MBC devices, this implies that costs of flash testing are increasing.
It would therefore be highly advantageous to have a systems and methods for testing flash memory devices in a cost effective manner. It would highly advantageous to have novel flash memory devices that can conveniently be tested in a cost-effective manner.